Résumé:

The presence of a liquid condensed phase inside the nanometrically sharp cracks in glasses plays an important role in the physics and chemistry of crack propagation, as well as in many industrial problems related to the strength and life time of glass products. The slow crack propagation in glasses is commonly explained by the stress-corrosion theory: water molecules that move in the crack cavity effectively reduce the bond strength at the strained crack tip and thus enhance the sub-critical crack propagation velocity, which is ruled by the rate of a thermally activated chemical reaction. However, the details of the local environmental condition at the crack tip in moist air are very complex and still need careful investigation. In this talk, I will present direct evidence of the presence of a submicrometric liquid condensate at the crack tip of a pure silica glass during very slow propagation. These observations are based on in-situ AFM phase imaging techniques applied on DCDC glass specimens in controlled atmosphere. Several macroscopic measurements of anomalous crack propagation in glasses had hinted at the presence of such a condensate, yet the direct observation of this phenomenon was not possible until recently due to the difficulty of real-time measurements at such small scale. The mechanical and chemical consequences of such a condensate will be discussed in the frame of stress-corrosion theory.